Spring steel wire rod excellent in pickling performance

Abstract

A spring steel wire rod according to the present invention is a spring steel wire rod wherein: the spring steel wire rod contains C: 0.35% to 0.7% (by mass, the same is hereunder applied), Si: 1.5% to 2.50%, Mn: 0.05% to 1.0%, Cr: 0.05% to 1.9%, Cu: 0.05% to 0.7%, Ni: 0.15% to 0.8%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding 0%); and the balance being Fe and unavoidable impurities, wherein the ratio of Si to Cu (Si / Cu) is in the range of 4 or more; and the difference between the concentration of Cu in the surface layer and the concentration of Cu in the steel is 0.50% or less and the difference between the concentration of Ni in the surface layer and the concentration of Ni in the steel is 1.00% or less. The present invention makes it possible to improve pickling performance in a spring steel wire rod containing Si abundantly and further Cr, Cu, and Ni.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a spring steel wire rod excellent in pickling performance, and more specifically a technology for improving pickling performance of a spring steel wire rod containing Si abundantly and further Cr, Cu, and Ni. A spring steel wire rod according to the present invention is favorably used for: a valve spring used for the engine of an automobile or the like; a clutch spring; a brake spring; a suspension spring such as a stabilizer spring or a torsion bar spring; and other applications. BACKGROUND OF THE INVENTION [0002] The chemical composition of a spring steel used for a valve spring, a suspension spring, or the like is stipulated in JIS G3565 to JIS G4801 for example and an appropriate steel type is used in accordance with the type of spring design. In recent years, as the size and weight of a spring reduce with the reduction of exhaust gas and fuel consumption, the design stress of the spring increases and it is desired t...

AI Technical Summary

Benefits of technology

The present invention provides a spring steel wire rod with improved pickling performance, especially when containing Si, Cr, Cu, and Ni. The wire rod has a specific composition with low amounts of P and S, and a ratio of Si to Cu of 4.0 or more. The difference between the concentration of Cu and Ni in the surface layer and the steel is very small. The wire rod also contains optional elements such as V, Ti, and Nb. The spring fabricated from this wire rod has excellent surface properties and is easy to pickle.

Problems solved by technology

), a dense melt phase wherein those oxides are intricately interwound with each other is formed and then, when it is further heated to a temperature of 1,200° C. or higher, Cr intrudes into the aforementioned melt phase and the fayalite, condenses at the interface with the base steel, and forms a condensed layer (the detail will be described later) The Cr once condensed is very hard to remove through the succeeding processes and deteriorates pickling performance.

When the pickling performance deteriorates, scale remains after pickling treatment, hence the adhesiveness with a lubricant applied on the surface (used for surface coating treatment) lowers, and wire breakage may undesirably occur during drawing.

In this case however, since the time of dipping in a pickling solution increases, the attack to the base steel by acid intensifies, the surface roughness lowers, and finally the durability comes to deteriorate even in the air.

Further, since a part of hydrogen generated during the attack to the base steel by acid diffuses rapidly in the steel and is absorbed therein, the amount of stored hydrogen increases, that causes the embrittlement (hydrogen embrittlement) of the steel material, and wire breakage occurs during drawing in some cases.

Such a phenomenon is called liquid metal melt embrittlement and it is understood that the phenomenon resultantly deteriorates pickling performance.

As stated above, although a spring steel wire rod containing Si abundantly and Cr, Cu, and Ni is used mostly in order to improve proof stress and corrosion fatigue resistance, there is concern that, in the case of such a spring steel wire rod, Cu concentrates at the interface between scale (particularly fayalite) and the base steel and hence pickling performance deteriorates.

However, a method for sufficiently improving the pickling performance of such a spring steel wire rod is not disclosed yet.

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